1. Field of the Invention
The present invention relates to process parameter sensors, operating methods and computer program products, and more particularly, to vibrating conduit parameter sensors, operating methods and computer program products.
2. Statement of the Problem
Coriolis effect mass flowmeters are commonly used to measure mass flow and other information for materials flowing through a conduit. Exemplary Coriolis flowmeters are disclosed in U.S. Pat. Nos. 4,109,524 of Aug. 29, 1978, 4,491,025 of Jan. 1, 1985, and Re. 31,450 of Feb. 11, 1982, all to J. E. Smith et al. These flowmeters typically include one or more conduits having a straight or a curved configuration. Each conduit may be viewed as having a set of vibration modes, including, for example, simple bending, torsional, radial and coupled modes. In a typical mass flow measurement application, each conduit is driven to oscillate at resonance in one of its natural modes as a material flows through the conduit. The vibration modes of the vibrating, material-filled system are effected by the combined mass and stiffness characteristics of the conduits and the characteristics of the material flowing within the conduits.
A typical component of a Coriolis flowmeter is the drive or excitation system. The drive system operates to apply a periodic physical force to the conduit that causes the conduit to oscillate. The drive system typically includes at least one actuator mounted to the conduit(s) of the flowmeter. The actuator may comprise one of many well known electromechanical devices, such as a voice coil device having a magnet mounted to a first conduit and a wire coil mounted to a second conduit, in an opposing relationship to the magnet. A driver typically applies a periodic, e.g., a sinusoidal or square wave, drive signal to the actuator coil. The periodic drive signal causes the actuator to drive the two conduits in an opposing periodic pattern.
When there is effectively "zero" flow through a driven flowmeter conduit, points along the conduit tend to oscillate with approximately the same phase or a "zero-flow" phase with respect to the driver, depending on the mode of the driven vibration. As material begins to flow from an inlet of the flowmeter, through the conduit and out of an outlet of the flowmeter, Coriolis forces arising from the material flow tend to induce phase shifts between spatially separate points along the conduit, with the phase on the inlet side of the conduit generally lagging the actuator and the phase on the outlet side of the conduit generally leading the actuator The phase shift induced between two locations on the conduit is approximately proportional to the mass flow rate of material through the conduit.
Unfortunately, the accuracy of measurements obtained using conventional phase shift or time delay methods can be compromised by nonlinearities and asymmetries in the flowmeter structure, as well as by vibration introduced into the flowmeter structure by external sources such as pumps. These effects may be reduced, for example, by using balanced mechanical designs that reduce the effects of external vibration and by using frequency domain filtering to remove frequency components associated with undesirable vibrations. However, mechanical design approaches may be constrained by geometric considerations, and frequency domain filtering may be ineffective at removing unwanted vibrational energy that occurs at or near resonant frequencies of interest such as the drive frequency used to excite the conduit.
One type of error commonly encountered in mass flow rate measurement applications is "zero offset." As mentioned above, mass flow rate measurements typically involve determining a phase or time difference between motion signals produced by transducers on the sensor conduit structure. Zero offset represents a bias or offset in these phase or time differences measurement, such that a zero mass flow rate does not produce a zero phase or time difference.
To reduce zero offset, error, conventional mass flow measurement techniques typically measure zero offset as a phase or time difference between motion signals measured under a controlled zero mass flow condition. Phase or time difference measurements conducted under other flow conditions are then compensated according to the measured zero-flow phase or time difference to produce more accurate results.
These techniques have potential disadvantages, however. Changes in process temperatures or sensor mounting conditions may cause the zero offset to drift over time and lead to measurement errors. To compensate for this drift, it may be necessary to periodically re-measure zero offset. This may be inconvenient, as conventional zero offset compensation techniques may require that flow be stopped to generate an updated zero offset measurement.